Reconfigurable photonic band gap device

ABSTRACT

A reconfigurable photonic band gap device having a substrate and a crystal membrane with a lattice structure having its ends attached to a substrate so that a midportion of the lattice membrane is spaced upwardly from the substrate and forms a chamber therebetween. A bridge is disposed in the chamber between and separated from both the lattice membrane midportion and the substrate. At least one post is attached to the bridge and aligned with at least one hole in the lattice so that movement of the bridge relative to the lattice varies the degree of insertion of the post relative to its associated hole to vary the photonic band gap behavior of the device.

RELATED APPLICATION

This application claims priority of U.S. Provisional Patent ApplicationSer. No. 60/550,846 filed Mar. 5, 2004, which is incorporated herein byreference.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the United States Government.

FIELD OF THE INVENTION

The present invention relates generally to photonic band gap devicesand, more particularly, to a reconfigurable photonic band gap device.

DESCRIPTION OF RELATED ART

Photonic band gap (PBG) structures typically comprise a crystal latticewhich disallows the propagation of light of certain wavelengths throughthe lattice. However, when a defect line is created on the photoniccrystal structure, the PBG structure can be configured as an opticalwaveguide which facilitates the propagation of light of certainwavelengths. Variations in the physical dimension of the defect linechange the dispersion properties of the waveguide to facilitate orenable light propagation. Therefore, a photonic integrated circuit canbe built with such a PBG structure.

A primary disadvantage of the previously known photonic crystal basedphotonic integrated circuits is that such photonic integrated circuitsare formed with fixed lattice structures and fixed defect lines and,accordingly, exhibit a fixed functionality. Consequently, in order toform a photonic system having different functionalities, it is necessaryto use different types of photonic integrated circuits interconnectedtogether. Each different photonic integrated circuit, of course,requires its own set of fabrication masks, etc., thus increasing theoverall cost for the individual photonic integrated circuits and thecorresponding overall cost of the photonic system.

SUMMARY OF THE INVENTION

The present invention provides a reconfigurable photonic band gap devicewhich can be selectively configured to operate as any of a plurality ofdifferent photonic functionalities, such as waveguides, switches, timedelays, intensity modulators, phase modulators, resonators, etc.

In brief, the reconfigurable photonic band gap device of the presentinvention comprises a semiconductor crystal substrate which may be ofany conventional composition, such as a gallium arsenide substrate. Atwo-dimensional photonic crystal lattice membrane with a plurality ofholes has its ends attached to the substrate so a midportion of thelattice membrane is spaced upwardly from the substrate and forms achamber between the midportion of the lattice membrane and thesubstrate.

A bridge is disposed in the chamber between the substrate and thephotonic crystal lattice membrane and is positioned closely adjacent thelattice membrane with a small distance of about one wavelength of theoperating band gap light wavelength. The portion of the bridge which isaligned with the lattice membrane is separated from both the latticemembrane and the substrate and thus movable relative to the latticemembrane. The bridge is constructed of a crystal material compatiblewith the substrate and crystal lattice membrane, such as an alloy of thesubstrate.

At least one post extends from the bridge and is aligned with at leastone hole in the lattice. Preferably, a plurality of linearly alignedposts extend from the bridge wherein each post is aligned with one holein the lattice.

The bridge is movable relative to the lattice to vary the degree ofinsertion of each post into its corresponding hole in the latticemembrane. In order to move the bridge relative to the lattice, a firstelectrical contact is formed on the substrate and a second electricalcontact is formed on the bridge. Consequently, by varying the voltagepotential between the electrical contacts, the bridge flexes away fromand towards the lattice as a result of coulombic attraction between thebridge and the substrate.

The present invention also discloses a method of making a reconfigurablephotonic band gap device. The method includes the step of forming awafer having a crystal substrate, a sacrificial layer overlying thecrystal substrate, a bridge layer overlying the sacrificial layer, and acrystal membrane overlying the bridge layer. All of the wafer layers andmembrane are made from compatible semiconductor crystalline material,such as gallium arsenide and/or alloys thereof.

A lattice is first etched through the crystal membrane layer usingconventional electron beam lithography and reactive ion plasma etchingso the layer has a plurality of lattice holes. The size, shape andspacing of the holes will vary as a function of the desired opticalwavelength and the characteristics of the photonic band gap device. Inaddition, a portion of the crystal membrane is left intact within aplurality of linearly aligned holes in the lattice to form a defectline.

Thereafter, a portion of the sacrificial layer beneath the lattice isremoved to form an internal chamber or space within the wafer. Anyconventional means, such as an selective wet chemical etch, may beutilized to remove the sacrificial layer.

A portion of the bridge layer sufficient to separate the bridge layerfrom the crystal membrane is then removed by another selective wetchemical etch. Following this chemical etch at least one post comprisinga portion of the bridge layer as well as a portion of the membraneextends into a registry lattice hole in the crystal membrane layer.

Electrical contacts are then attached to both the bridge layer as wellas the substrate. In use, a voltage differential applied between thebridge and the substrate through the contacts flexes the bridge due tocoulombic attraction. In doing so, the bridge with its attached post(s)moves toward and away from the membrane in order to vary the degree ofinsertion of the post(s) within their corresponding lattice holes.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be had uponreference to the following detailed description when read in conjunctionwith the accompanying drawing, wherein like reference characters referto like parts throughout the several views, and in which:

FIG. 1 is an elevational simplified view illustrating a preferredembodiment of the photonic band gap device of the present invention;

FIG. 2 is a sectional view taken substantially along line 2—2 in FIG. 1;

FIG. 3 is a view similar to FIG. 2, but illustrating the bridge in aflexed state;

FIG. 4 is a diagrammatic enlarged view illustrating a portion of thepresent invention;

FIG. 5A is a top view illustrating a first step in making the photonicband gap device of the present invention;

FIG. 5B is a sectional view taken along line 5B—5B in FIG. 5A;

FIG. 6A is a top view of a second step in making the photonic band gapdevice of the present invention;

FIG. 6B is a sectional view taken along line 6B—6B in FIG. 6A; and

FIG. 7 is a sectional view similar to FIG. 6B but illustrating a furtherstep in making the photonic band gap device of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

With reference first to FIGS. 1 and 2, a preferred embodiment of thephotonic band gap device 20 of the present invention is shown andcomprises a semiconductor crystal substrate 22. Any conventionalmaterial, such as gallium arsenide, may be used as the substrate 22.

A photonic crystal membrane 24 having a plurality of holes 26 which forma two-dimensional lattice structure 25 is attached to the substrate 22.Furthermore, the lattice structure 25 is positioned between an opticalinput 28 and an optical output 30 for the photonic device 20.

The lattice holes 26 may be of any conventional size and shape for aphotonic band gap structure. However, as shown in the drawing, thelattice holes 26 are generally hexagonal in shape although other shapesmay alternatively be used without deviation from the spirit or scope ofthe present invention. Furthermore, the distance between the centers ofadjacent holes in the lattice structure 25, the size of the holes aswell as the thickness of the wall separating the holes 26 from eachother determine the band gap wavelength for the photonic band gap device20 in the well known manner.

Referring now particularly to FIG. 2, a bridge 32 is attached to thesubstrate 22 and is positioned between the lattice structure 25 and thesubstrate 22. The bridge 32 may be either attached at both ends to thesubstrate 22 or may alternatively be of a cantilever design.

The bridge 32 is made of a semiconductor crystal material compatiblewith the substrate 22, such as an alloy of the substrate 22. As bestshown in FIGS. 2 and 3, a chamber 34 is formed in between the bridge 32and the substrate 22 thus allowing the bridge 32 to flex between theposition shown in FIG. 2 and the position shown in FIG. 3.

At least one, and preferably a plurality of posts 36 arranged in ageometric pattern, such as a line, extend upwardly from the bridge 32between the optical input 28 and the optical output 30. Each post 36 isaligned with a corresponding hole 26 in the lattice structure 25.Consequently, as best shown in FIGS. 2–3, as the bridge 32 moves betweenthe position shown in FIG. 2 and to the position shown in FIG. 3, thedegree of insertion of each post 36 relative to its associated hole 26will vary as a function of the deflection of the bridge 32.

As best shown in FIG. 4, each post 36 preferably includes a stem 38 anda cap 40. The stem 38 is integral with and of the same material as thebridge 32, while the cap 40 is formed from a portion of the latticemembrane 24. The cap 40 is attached to the upper or free end of eachstem 38, and it is the movement of the cap 40 relative to the latticehole 26 which varies the degree of insertion of the posts 36 relative tothe lattice holes 26.

With reference again to FIGS. 2 and 3, in order to deflect the bridge 32relative to the lattice structure 25, a pair of electrical contacts 42and 43 (illustrated only diagrammatically) are attached to the bridge 32and substrate 22 in any conventional fashion. A voltage potentialapplied to the contacts 42 and 43 will cause the bridge 32 to deflectrelative to the crystal membrane 24 due to the coulombic attractionbetween the bridge 32 and the substrate 22. The magnitude of thatvoltage potential, in turn, determines the amount of the deflection ofthe bridge 32 and thus the degree of insertion of the posts 36 intotheir corresponding holes 26 in the lattice structure 25.

In the well known fashion, the posts 36, when inserted into theircorresponding holes 26 in the lattice structure 25, form a defect linewhich enables light propagation in single mode through the latticestructure 25. Conversely, when the posts 36 are retracted from theircorresponding holes 26 in the lattice structure 25, the latticestructure 25 forms a band gap at the operating wavelength of the bandgap device. However, by varying the voltage potential across theelectrical contacts 42 and 43 and thus the degree of insertion of theposts 36 into the lattice holes 26, the band gap device 20 may beselectively configured as different optical devices including, but notlimited to, a transmission waveguide, switch, intensity modulator, phasemodulator, true time delay line and the like.

With reference now to FIGS. 5A and 5B, a method for making the photonicband gap device 20 of the present invention will now be described indetail. As best shown in FIG. 5B, a wafer 60 is first formed having thesemiconductor crystalline substrate 22. Any conventional material, suchas gallium arsenide, may be used as the substrate 22.

A sacrificial layer 64 is then formed on one side of the substrate 22 sothat the sacrificial layer 64 overlies and covers the substrate 22. Thesacrificial layer 64 is made of a crystalline material compatible withthe substrate 22, but which can be selectively etched by a chosen wetchemical that does not etch the membrane layer and/or substratematerial, such as aluminum gallium arsenide (Al_(x1)GaAs) where x1 is inthe range of 0.45 to 1.0. Any conventional means, such as molecular beamepitaxial (MBE) growth, may be used to form the sacrificial layer 64.

A bridge layer 66 made of a semiconductor crystalline materialcompatible with the sacrificial layer 64 is then formed so that thebridge layer 66 overlies and covers the sacrificial layer 64. The bridgelayer 66 may be formed of any compatible material, such as an alloy ofthe substrate 22, and preferably comprises aluminum gallium arsenide(Al_(x2)GaAs) where x2 is in the range of 0.3 to 0.6. The aluminumcontent of the bridge layer 66 is at least ten percent less than thealuminum content of the sacrificial layer 64 for a reason to besubsequently described. In addition, the bridge layer 66 may be formedby any conventional means, such as MBE growth.

The crystal membrane 24 then overlies the bridge layer 66. The membrane24 is of a crystalline material compatible with the bridge layer 66 andis preferably made of gallium arsenide. The thickness of the membrane 24is determined by the waveguiding mode of the operating light. Forexample, the bridge layer 66 is preferably in the range of 1 to 6microns whereas the crystal membrane 24 is approximately 0.3 microns inthickness for single mode waveguide with 1550 nm operation lightwavelength.

With reference now to FIGS. 5A and 5B, the lattice structure 25 with itsholes 26 is formed through the membrane 24 by any conventional method,such as electron beam lithography and reactive ion plasma etching. Theshape, size and spacing of the holes 26 relative to each other definethe band gap characteristics of the photonic device 20. In addition,portions of the membrane 24 which form the caps 40 of the posts 36 areleft intact after the plasma etch.

In addition to forming the lattice holes 26 during the plasma etch,additional portions 74 and 76 of the membrane 24 adjacent and alongsidethe lattice structure 25 are also removed from the membrane 24 in orderto isolate the sides of the lattice structure 25. Similarly, portions 76adjacent one end of the lattice structure 25 and portions 78 adjacentthe other end of the lattice structure 25 of the membrane 24 are alsoremoved from the membrane 24 during the plasma etch. By removing theportions 76 and 78 from the membrane 24, the remaining membrane 24 inbetween the removed portions 76 of the membrane 24 form the opticalinput 28 to the lattice structure 25 while, similarly, the remainingmembrane 24 between the removed portions 78 of the membrane 24 form theoptical output 30 for the photonic device 20.

With reference now to FIGS. 6A and 6B, using standard photoresistmaterial 84 and standard photolithography procedures, a first elongatedtrench 86 is formed through the photoresist material 84 across the inputend of the lattice structure 25. Similarly, a second elongated trench 88is also formed across the lattice structure 25 adjacent the output endof the lattice structure 25.

A chosen acid as chemical etcher is then applied to the trenches 86 and88 under the membrane layer 24 though the holes 26. The acid is selectedso that both the substrate 22 and crystal membrane 24 are substantiallyimpervious to the acid while the crystalline alloys used to form boththe sacrificial layer 64 and bridge layer 66 are susceptible to theacid. For example, hydrofluoric acid is preferably used to perform theacid etch where the substrate 22 and membrane 24 comprise galliumarsenide while the sacrificial layer 64 and bridge layer 66 comprisealuminum alloys of gallium arsenide.

Since the sacrificial layer 64 has a higher aluminum content than thebridge layer 66, preferably at least 10% greater, the acid introducedinto the trenches 86 and 88 effectively and rapidly removes thesacrificial layer 64 from the wafer 60 in the area directly aligned withand below the lattice structure 25. This, in turn, creates a void 90 inthe wafer 60 beneath the bridge layer 66 and in alignment with thelattice structure 25. Conversely, the bridge layer 66 is lesssusceptible to the acid due to its lower aluminum content so that, asbest shown in FIGS. 2 and 3, the bridge 32 remains substantially intactbetween the trenches 86. Following this acid etch, the photoresistmaterial 84 is removed in the conventional fashion.

With reference to FIG. 7, a second and relatively shorter acid etch isthen applied to the lattice structure 25. In doing so, the acid flowsthrough the holes 26 in the lattice structure 25 and thus etches aportion of the bridge layer 66 beneath the membrane 24 and around thecaps 40 of the posts 36. This acid etch, however, is carefullycontrolled so that the etch time and acid concentration is sufficient toseparate the bridge layer 66 from the membrane 24, as shown in FIG. 4,while retaining the attachment of the posts 36 to the bridge layer 32.The lattice structure 25 is selected so that the holes 26 and posts 36are much larger in diameter than the wall thickness of the latticestructure 25 between adjacent holes 26. Consequently, although someundercutting of the stems 38 of the posts 36 will occur during this acidetch, the post stems 38 remain substantially intact despite completeundercutting of the bridge layer 66 beneath the walls of the latticestructure 25.

Thereafter, electrical contacts are applied to the bridge layer 66through a contact pad and substrate 22 in any conventional fashion.

From the foregoing, it can be seen that the present invention provides areconfigurable photonic band gap device, and a method of making thesame, which overcomes all of the disadvantages of the previously knowndevices. More specifically, by controlling the degree of insertion ofthe posts 36 into their corresponding lattice holes 26, the photonicdevice of the present invention can be configured to perform any of anumber of different photonic functionalities.

Having described my invention, however, many modifications thereto willbecome apparent to those skilled in the art to which it pertains withoutdeviation from the spirit of the invention as defined by the scope ofthe appended claims.

1. A reconfigurable photonic band gap device comprising: a substrate, aphotonic band gap crystal membrane having a two-dimensional latticestructure with a plurality of holes, said lattice structure having endsattached to said substrate and a midportion spaced upwardly from saidsubstrate and forming a chamber between said lattice membrane midportionand said substrate, a bridge disposed in said chamber between andseparated from said lattice membrane midportion and said substrate, atleast one post extending from said bridge, said at least one post beingaligned with at least one hole in said lattice structure, wherein saidbridge is movable relative to said lattice structure to vary the degreeof insertion of said at least one post relative to said at least onelattice hole.
 2. The invention as defined in claim 1 wherein said atleast one post comprises a plurality of posts, each said post beingaligned with a corresponding hole in said lattice.
 3. The invention asdefined in claim 2 wherein said posts are arranged in a geometricpattern in the photonic crystal membrane.
 4. The invention as defined inclaim 1 and comprising means for moving said bridge relative to saidlattice.
 5. The invention as defined in claim 4 wherein said movingmeans comprises means for creating a voltage differential between saidbridge and said substrate.
 6. The invention as defined in claim 1wherein said crystal membrane comprises gallium arsenide.
 7. Theinvention as defined in claim 1 wherein said substrate comprises galliumarsenide.
 8. The invention as defined in claim 1 wherein said bridgecomprises an aluminum gallium arsenide alloy.
 9. The invention asdefined in claim 8 wherein each post comprises a portion of said bridgeand a portion of said crystal membrane.
 10. The invention as defined inclaim 1 wherein said lattice holes are hexagonal in shape.